Laser annealing (also called laser spike annealing, laser thermal annealing, laser thermal processing, etc.) is used in semiconductor manufacturing for a variety of applications, including for activating dopants in select regions of devices (structures) formed in a semiconductor wafer when forming active microcircuits such as transistors and related types of semiconductor features.
One type of laser annealing involves the formation of a line-shaped intensity profile that is scanned over the semiconductor wafer by moving the line image, moving the semiconductor wafer, or a combination of these two movements. The line image is scanned in a “scan direction,” which is perpendicular to its long axis. Some spatial variation in the intensity in the line image along the scan direction (i.e., the short axis of the line image) can be tolerated since the non-uniformities are averaged out as the line image moves over the semiconductor wafer. On the other hand, the spatial variation of the intensity profile in the “cross-scan” direction of the line image needs to be tightly controlled to achieve consistent annealing results over the scan path of the line image.
In forming the line image, a pulsed laser beam that has a Gaussian intensity profile needs to be shaped into a flat-top or super-Gaussian intensity profile. This can be done through beam homogenization, which can be performed using a micro-lens array or a light pipe to divide the incident beam into multiple wavelets and then recombining the wavelets. The overlap of the multiple wavelets creates a macroscopically uniform beam. However, when the laser beam is coherent, there can still be micro-scale beam profile non-uniformities due to interference effects, such as speckle.
FIG. 1A is a plot of an example prior art intensity profile I(x) versus x for an example line image formed at an image plane where a wafer surface is located. The plot of FIG. 1A schematically illustrates example micro-scale intensity variations Im(x) that can be seen when the intensity profile is viewed at small scales, i.e., with a range typically between a fraction of a micron to tens of microns, as compared to macro-scale variations, which are measured in the range from about one millimeter to tens of millimeters. These micro-scale intensity variations are difficult to mitigate, especially when short light pulses are used in the annealing process.